Evaluation of CZT gamma cameras for human SPECT and small FOV imaging

Objectives: The direct-conversion semiconductor CdZnTe (CZT) gamma photon detector has been used in several molecular imaging applications, including both small-animal and human SPECT, cardiac SPECT, and molecular breast imaging (MBI). Integration of pixelated CZT detectors with advanced ASICs and readout electronics improves system performance. Our objective was to build both a small FOV gamma camera and a general-purpose human SPECT gamma camera and to evaluate their cost and performance for clinical applications.

Methods: We designed a modular pixelated gamma detector (4.4 cm square with 2 mm pixel pitch) and tiled it in two array sizes: a small FOV camera (8.8 cm square, 44 x 44 pixels) and a whole-body SPECT camera (39.6 cm x 52.8 cm, 198 x 264 pixels) that we mounted on a refurbished SPECT gantry for demonstration. We tested both parallel hexagonal- and square-hole collimators, where the square holes are aligned with pixels.

Results: We measured energy resolution of 3.6% FWHM at 140 keV, intrinsic flood field uniformity of ±0.8% integral and ±0.4% differential, system spatial resolution at 10 cm with an LEHR collimator of 6.8 mm and 7.5 mm without and with scatter, and non-paralyzable detector count rate performance (in the 8.8 cm square camera) of 425 k cps at 20% loss and 1.9 M cps maximum observed. These metrics are significantly better than scintillator SPECT systems. The cost of CZT gamma cameras has decreased dramatically in the past decade and this trend will continue with economies of scale. Excellent energy resolution enables better scatter rejection and image contrast, further enhanced by excellent uniformity. Spatial resolution is improved, even with hexagonal-hole collimators, but registered square-hole collimators provide significantly better resolution and a boost in sensitivity which can lower dose and/or exam time. The non-paralyzable CZT cameras enable new first-pass and pharmacokinetic studies. Conclusion: Advances in detectors and collimators have significantly improved efficiency of CZT-based molecular imaging systems. With CZT cost and performance improving, affordable whole-body CZT general purpose SPECT is expected to enable precision medicine applications.